Halogen incandescent lamp--reflector combinations require that the light source of the lamp is accurately positioned with respect to the reflector with which it is to be used. Usually, the reflectors are formed with an opening which terminates in a reference surface or plane, against which a corresponding lamp reference surface or plane can be engaged. When the two reference surfaces are in contact, a light source which meets specifications is accurately placed with respect to the reflector. The lamp itself may have two or three terminals, to provide both for high-beam and low-beam energization, with respective filaments or filament portions being located in predetermined positions with respect to the reference surface so that, when the lamp is incorporated in the reflector, the light emitted will be in accordance with predetermined and standardized light distribution patterns, as required by highway safety regulations.
A lamp of this type is described in U.S. Pat. No. 4,463,278, Kosmatka et al. This lamp has a three-element base which includes a metallic holder structure which engages the pinch seal of a halogen incandescent lamp bulb by springy or resilient projections or tongues. The holder extends, in part, within a tubular plastic sleeve. The edge of the sleeve remote from the bulb is seated directly on a plane surface of a bottom portion, likewise of plastic and, before being attached to the bottom portion, can be laterally shifted to provide for optical adjustment with respect to the bottom portion. It can also be rotated.
It has been found that the arrangement is suitable only with plastic bases since, if all-metal or primarily metal bases are used, the substantially thinner walls do not permit the required accuracy of adjustment. The engagement surfaces, due to the thin-walled metal structures, would be insufficiently large. The coupling between the metallic holder and the adjustment sleeve of plastic on the one hand and of the adjustment sleeve on the bottom portion, on the other, require different connecting technologies, one of metal-to-plastic and the other of plastic-to-plastic. The two different connecting technologies cause problems in manufacture, since change-over from one connecting technology to another is complex and time-consuming.
It has been found, in further use, that the freedom of adjustment of the adjustment sleeve with respect to the holder is unduly limited. The sleeve is formed as a straight-tubular element, in which the holder must be accurately fitted in order to obtain, eventually, a good coupling or bonding connection. This requires close tolerances, difficult to achieve with plastic elements and, additionally, does not permit tipping of the holder as an additional freedom of adjustment. Actually, in use, it has been found that the type of holder structure there disclosed did not find commercial acceptance.
The referenced U.S. Pat. No. 5,010,272, Eckhardt et al, describes a combination metal-ceramic holder structure. A metallic holder element can be coupled to a ceramic core element which is essentially immune to high-temperature degradation and provides for excellent insulation and separation of current carrying elements.